1. Technical Field of the Invention
The present invention relates to a plastic molding die for molding a plastic material.
2. Description of the Related Art
As a plastic molding die for manufacturing a plastic lens with compression molding, using a plastic material, there is a plastic molding die which is configured so that a cavity is formed between each connection mounted on each main die of the plastic molding die when the each main die, an upper main die and a lower main die, is combined to each other.
In the plastic molding die, a plastic material is put in a space between the connections mounted on each main die when the each main die is separated to each other, and the plastic material is processed with compression molding within the cavity by combining the upper main die and the lower main die after heating up the material to a temperature equal to or higher than a glass-transition temperature for softening the plastic material. This is disclosed in, for example, Japanese Laid-Open Patent Application No. 2000-25120 (paragraph [0007]-[008], and FIG. 1).
Here, the plastic material is taken out from the cavity after cooling the material to a temperature equal to or less than the glass-transition temperature because the softened plastic material is likely to be deformed if a temperature of the plastic material is higher than the glass-transition temperature when a molded material is taken out from the cavity. A plastic material drastically changes its volume at around the glass-transition temperature. Then, when the plastic material is cooled within the cavity, since the material shrinks within the cavity, thereby the material separates from an inner surface of the cavity, a molding accuracy of the plastic material becomes poor.
Therefore, in the plastic molding die described in the above, the die is configured so that the plastic material is maintained to be contacted with the inner surface of the cavity by displacing the connection toward the cavity, thereby maintaining a stress of the plastic material when the molded plastic material is cooled within the cavity. Namely, a volume of the cavity is reduced by displacing the connection according to shrinkage of the plastic material when the material is cooled.
However, in the above plastic molding die, a driving unit for displacing the connection must be built in the main die as well as a necessity of a control unit for displacing the connection according to a volume change of the plastic material. As a result, the die becomes complex.
In addition, regarding a product which is molded with the above method, a dimension of the product in a displacement direction of the connection may be varied. Therefore, the method can not be applied to a product of which dimension in the displacement direction is important.
It is therefore requested for the present invention to provide a plastic molding die which can solve the aforementioned issues, and can improve a molding accuracy of a plastic material without making the molding die complex.